This article shows that the final-particle development from coordinating conjunctions and that from subordinating conjunctions in Japanese can be best described as ‘adaptation after exaptation’ and ‘successive adaptations’, respectively. Whereas both types of developments are comparable in that conjunctions ordinarily occupying an inter-clausal position become sentence-final particles, they differ in how the conjunctions come to occur in the sentence-final position. Subordinating conjunctions are left behind in the position as a result of an adaptation, while coordinating conjunctions are coopted or exapted into the position. Although some facets of the Japanese final-particle development from conjunctions can be treated as instances of analogy or reanalysis, these notions do not explain the different pathways through which those conjunctions reach the sentence-final position. The paired notions, adaptation and exaptation, are claimed to be useful and necessary for linguistic studies in terms of parallelism between biological and linguistic evolution, permissive environment, and non-metaphorical explanation.